These Tri-Boro open style steel shelving units are constructed with four angle posts so each unit is a stand alone shelving unit allowing for greater flexibility, as opposed to Beaded or T-Post style units that must be built in place and dissassembled to move. Open units utilize bolt in side and back braces. Starter units have X-bracing on both sides, while Add-On units only have X-bracing on one side, as they "share" the common side brace with the unit next to it. For example, a run of three units along a wall would have one Starter and two Add-Ons.

This economy steel shelving does a great job. Compared to more expensive brands I am very happy with it's cost and it is still a true industrial grade shelf as opposed to the commercial grade cheap stuff at the local Lowes or Home Depot.Packaging could be better, otherwise it would have gotten 5 stars.